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Speedo reading to fast.


Sierra

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A while back, I did a transmission swap on my 78 B210 (from automatic to a 5 speed dog-leg). 

After that my speedo has been clocking me about 5-7 miles faster on the highway.

I know I can change the speedo pinion gear in the transmission for something with less or more teeth in order to fix the problem.

My issue is that I cannot find a proper speedo pinion gear.

My question is the following.

What if I change the gauges cluster? Would that fix the issue?

I am assuming that the speedometer gauge is calibrated different in the automatic car and the manual 5 speed car.

Thanks in advance for you input.

 

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5-7 miles per hour at exactly what speed? Find out what it is at 50 and 2X that is the percent.

 

For example 55 MPH at 50 is 10%. If you had a 17 tooth gear, divide that into a 19 tooth and you get just under 12% which would slow you half that (6 MPH) at 50 or 49... which is damn close. 

 

The automatic has the correct number of teeth on the speedo pinion to match the differential ratio. Only way to fix is change the differential or the gear.

 

Messing with the speedometer? I don't know. Easier to change to the correct gear but not as easy to find one.

 

 

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10 hours ago, Sierra said:

A while back, I did a transmission swap on my 78 B210 (from automatic to a 5 speed dog-leg). 

After that my speedo has been clocking me about 5-7 miles faster on the highway.

I know I can change the speedo pinion gear in the transmission for something with less or more teeth in order to fix the problem.

My issue is that I cannot find a proper speedo pinion gear.

My question is the following.

What if I change the gauges cluster? Would that fix the issue?

I am assuming that the speedometer gauge is calibrated different in the automatic car and the manual 5 speed car.

Thanks in advance for you input.

 

 

Mike has the technical stuff covered so I'll just add this.....

First I doubt changing the speedo would help.... 

What color is your current speedometer gear?

Also check the length of the the entire pionion assembly.... I'm not sure if the dogleg transmissions had the longer or shorter pinion assembly..... the shorter are more commonly available ....

 

There is also a good check to do....

Drive 10 miles , use highway markers if you can, GPS will work if needed.....

Then record how many miles your odometer said..... if that's wrong then you need the speedo gear or differential changed..... if the odometer track 10 miles then the speedometer needs recalibrating 

 

For me I corrected my odometer but I'm still 5 mph off across the whole range, i need to calibrate the speedo to fix that part.....

 

I was gonna link a place I bought a ratio adaptor from but it looks like their websight is down or something....

They calibrate it to correct the error you tell them you have.... so for me I told them I traveled 10 miles but odometer said I went 13 and they built an appropriate box..... 

I'll try to find their website later....

 

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Thanks for all your input guys.

I was hoping for a quick fix.

Specially since I already have a gauge cluster from a manual B210.

I will take the pinion gear out and count the teeth and note the color of it.

I will also measure how fast it is reading at 50mpg on the highway.

Once I have that info I will share it here so the problem solving process can continue.

I appreciate the help.

Thanks

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1 hour ago, Sierra said:

Thanks for all your input guys.

I was hoping for a quick fix.

Specially since I already have a gauge cluster from a manual B210.

I will take the pinion gear out and count the teeth and note the color of it.

I will also measure how fast it is reading at 50mpg on the highway.

Once I have that info I will share it here so the problem solving process can continue.

I appreciate the help.

Thanks

Check the length too.... that will be important if you need to change it...

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  • 2 months later...
On 6/13/2019 at 7:12 AM, Duncan said:

I ran into this the other day looking for something else, and I bookmarked it.  It has most of the speedo pinion info and p/n's all in one place.

 

https://zparts.com/index.php/2016/10/16/speedo-gears-illustrated/

 

This works fine if you have an S-30 tire that is around 24.75" tall. The 620 about an inch shorter. The 720 up to 26.7" tall. The 510 had 23.2 inch tall tires. It is a good visual but may not work exactly as you hope for. Each tooth up or down adds or removes (very roughly) about 5-6% to your speedometer.

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Ok got the gear, changed it out, and 5 mph better as I figured but I’m still 15 off, so it’s defiantly the speedo.  Found a calibration thread somewhere.  Apparently the needle can just be moved.  Probably need to put the original gear back on.

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Did you ever do a 10 mile check to see how the odometer tracks?

 

I said this a few posts back....

 

Just be aware you can calibrate the speedometer and /or move the needle but that wont keep the odometer reading correctly....

 

Just depends on which one you want accurate....

 

 

 

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You can also find a speed trap sign and hold a steady speed of say 40MPH and see what the sign says.  That will give you an idea of how much your car is off at a given speed.  A general ratio is 1 mph per tire size increase up or down.  The B210 came out with 13" tires, going up to a 14" standard size tire means the speedometer will read 1MPH over for every 10 MPH speed, and 1 MPH downward for every 10MPH speed if dropping down to a 12" tire.

 

And yes I know it seems not every scientific but that general rule has worked for me for decades.

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In the extreme you can find a speedometer shop that can de-solder the copper cup on the speedometer needle shaft and change the position on the shaft so as to correct the eddy current drag and restore correct speed readings.  did that on my 411.

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A 13" and a 14" is just the rim diameter. Both can easily have the correct tire on it so they have the same over all diameter. The 710 came with 165/70R13s which are 22.09" diameter. If going to a 14" rim 160/60R14 tires are 22.03", and the speedometer stays the same. 

 

Tire diameter and speedometer. IF changing to a tire 1" taller diameter tire, adds about 2.2 MPH at ALL speeds.

22"....... 47.8MPH

23"....... 49.9MPH.... 2.1MPH

24"....... 52.0MPH.... 2.1MPH

25"....... 54.3MPH.... 2.3MPH

26".......56.5MPH..... 2.2MPH

 

Do the speed test at 25 MPH then multiply by 4X to get the % on 100MPH.

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  • 1 month later...

I had (and still have) the same problem after swapping from auto to 5-speed. If i remember correctly, there isn't an exact pinion that will work, so i went with the one that was closest. It seems like i may have used the purple one (not sure on that). What i ended up with is the speedo seeming to be just about right, but the odometer is exactly 1/10th of a mile short per mile. So when i calculate my gas mileage I add 10% to the total miles traveled. Oh, and i should note that i have 14" wheels.

 

 

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